Autoreactive MHC class II-reactive CD4 T lymphocytes play a critical role in the pathogenesis of Type 1 diabetes. Lysosomal cysteine proteinases, cathepsin (Cat) S, L, and B are implicated in antigen processing and presentation by MHC class II molecules to CD4 T cells. These enzymes participate in the proteolytic degradation of MHC class II associated invariant chain and in generation of antigenic peptides from self and foreign antigens. In this proposal, we will study non-obese diabetes prone (NOD) mice, which show MHC-dependent susceptibility to Type 1 diabetes analogous to that seen in humans. We have generated NOD mice deficient in Cat S, L or B by breeding the corresponding knockout mice onto the NOD background. Preliminary studies suggest that incidence of diabetes is significantly reduced in Cat S and B null mice as compared to heterozygous littermates. Reduced disease in Cat deficient mice can be due to: diminished antigen presentation by MHC class II molecules; diminished non-specific inflammation mediated by macrophages; reduced development of autoreactive CD4 T cells; increased activity of suppressor T cells; reduced autoantigen production in the islet _-cells; increased resistance of islet a cells to apoptosis. In this proposal, we will test all these hypotheses by investigating the role of cat B, S and L in spontaneous type I diabetes in the following specific Aims: 1) to characterize disease progression, MHC class II antigen processing and presentation, and CD4 T cell development in Cat-deficient and wild type control NOD mice; 2) to determine cellular mechanisms responsible for reduced diabetes in cathepsin-deficient NOD mice. This work will contribute to our understanding of the role of cathepsins in MHC class II antigen presentation, and provide new targets for downmodulating the immune responses leading to Type 1 diabetes.